1. Field of the Invention
The invention relates to exposing apparatus and method which are suitable for use in, for example, a video camera.
2. Description of the Prior Art
As shown in U.S. Pat. No. 5,093,716 by the applicant of the present invention, a video camera has an exposing apparatus comprising an iris, an electronic shutter, an AGC amplifier, and the like in order to make an input to a signal processing circuit constant. In FIG. 1, a circuit block diagram of a video camera in which the exposing apparatus is installed is shown. In FIG. 1, an object image is supplied to a CCD image pickup device 103 through a lens section 101 and an iris 102. In the CCD image pickup device 103, a light amount of the incident object image is photoelectrically converted. An output signal of the CCD image pickup device 103 is supplied to an AGC amplifier 104. An output signal of the AGC amplifier 104 is converted into a digital signal by an A/D converter 105 and supplied to a signal processing circuit 106 and a detecting circuit 107. The signal supplied to the signal processing circuit 106 is subjected to a predetermined signal process such as gamma correction or the like and, after that, the processed signal is output.
The digital signal supplied to the detecting circuit 107 is detected and, after that, it is supplied to a comparator 108. A signal at a reference level is supplied to the comparator 108 from a reference level signal generating section 109. Thus, the signal supplied from the detecting circuit 107 is compared with the reference signal. A detection output of the comparator 108 is supplied to a control amount operating circuit 111 through a loop filter 110. The control amount operating circuit 111 calculates an opening degree of the iris 102, a shutter speed of the CCD image pickup device 103, and a gain of the AGC amplifier 104 on the basis of the supplied signal. The calculation values calculated by the control amount operating circuit 111 are supplied to an iris driving circuit 112 to control the opening/closing operation of the iris 102, a shutter driving circuit 113 to control the shutter speed of the CCD image pickup device 103, and a D/A converter 114 to control the gain of the AGC amplifier 104.
There has been put into practical use a video camera with a hand vibration correcting function for detecting and correcting a hand vibration in the case where the hand vibration occurred, namely, a video camera having a hand vibration correcting function of the image processing type by a system in which a portion to be taken out as an image in an output signal of a CCD image pickup device as a photoelectric converting device is changed at any time in accordance with a hand vibration amount. For example, a device of the PAL system is used as a CCD image pickup device mentioned above. Due to this, after the hand vibration correction was performed to the photographed signal, an image signal of the NTSC system is output.
In such a video camera, in order to make the most of the effect of the hand vibration correction, it is necessary to improve a moving object resolution of an object whose image is formed on the CCD image pickup device. For this purpose, the electronic shutter speed of the CCD image pickup device is set to a speed higher than a standard speed (1/60 second in the NTSC system; 1/50 second in the PAL system), thereby improving the moving object resolution of the object. Generally, in consideration of the prevention of a flicker phenomenon which occurs in case of photographing under the irradiation of a fluorescent lamp, the electronic shutter speed is set to 1/100 second in the NTSC system and to 1/120 second in the PAL system.
On the other hand, the exposing apparatus controls the numerical aperture of the iris, electronic shutter speed, and gain of the AGC amplifier so that the image pickup signal level coincides with the exposure control reference value within a control range (a range from a control value in which each gain becomes maximum to a control value in which it becomes minimum is set to a control range). Therefore, in such an exposing apparatus, for example, in case of photographing the same object, even if the brightness (luminance level) of the object differs, a constant exposure amount is output so long as it lies within the exposure control range. However, when the brightness of the object changes, the brightness at which the user actually sees and feels obviously changes. Therefore, a difference occurs between the brightness of the video signal output of the video camera and the brightness which each user feels and the brightness of an object video image which is output to the monitor or the like differs from the actual brightness. Consequently, the brightness of the object video image of the final output becomes unnatural.
In the video camera as mentioned above, in the operating state of the hand vibration correcting function, the electronic shutter speed is set to 1/100 second or 1/120 second irrespective of the brightness of the object. Therefore, an exposure amount is always lost by only about -4.5 dB in the NTSC system and by only about -7.6 dB in the PAL system. Consequently, the minimum object illuminance of the video camera increases due to such a lost exposure amount and the performance is deteriorated with respect to a point of the operation guarantee illuminance range.